In the field of measurement of dimensions of a non-ferrous, electrically conducting object contactless methods are known which are based on the fact that such an object influences a magnetic field being applied to the object since an electric current is induced inside the object. The influence of the object on the magnetic field depends mainly on the geometrical dimensions and the resistivity of the object. Accordingly, if the resistivity is known, the geometrical dimensions can be identified by measuring the change in the magnetic field caused by the object.
In order to create a distinctive point in time where the objects influence on the magnetic field can be clearly observed, the EP0349528B1 suggests changing the excitation of the magnetic field considerably by switching the current supply off. The time elapsing between the switch-off and the decaying of the part of the magnetic field being present in the air gap between the measuring equipment and the object is measured. The geometrical dimensions of the object are then determined based on the length of the measured time. In the following, the decay of the magnetic field in the air gap is called primary decay.
The method proposed in EP0349528B1 was further improved in WO 01/01065 A1 to achieve a higher accuracy for objects with smaller dimensions. There, the measurement is not started directly after the excitation was changed but only after the primary decay is completed. After the primary decay only the part of the magnetic field inside the object remains which decays with a bigger time delay than the field in the air gap. The decay of the magnetic field in the object can be called secondary decay. The length of the secondary decay depends again on the geometric and resistive parameters of the object. Further embodiments of this method are described in WO 2005/064268 A1 and WO 2005/064269 A1.
The known methods all have in common that a period of time is measured and used to determine the geometrical dimensions of the object. This approach has a main drawback which especially affects the measuring accuracy of objects with small dimensions, like thin sheets. Since only two measurements are used to define the period of time, the initiation of the sampling of these two measurements becomes crucial. Only minor variations in the time to sample caused for example by varying delays in the signal processing circuitry may lead to significant differences in the measurement result. These differences increase with decreased dimensions of the object. Furthermore, the exact determination of the point in time when the change in the excitation of the magnetic field takes effect becomes more important if the object is smaller.
Therefore, it is an object of the current invention to provide a signal processing method and a unit for a dimension-gauging system which is especially applicable to objects with small dimensions, in particular to very thin sheets.